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Oh, the campaign provides an excellent opportunity to experience and appreciate the demons as well!
Cerberuses are truly the AI's favorite demon unit. For other factions, this unit is usually among the best, but for Inferno, they somehow made an exception.
So, the main goal in the early stages of the game is to ensure the survival of the doggies. For a warrior hero, the ideal choice is Reinforcements; for a mage, it's Restoration. Then, you can take Drain.
The doggies are also a brainy attacking unit. Heroism + Onslaught for the warrior, and for the mage, probably Inner Flame + Haste – this is the ideal combination to unleash the full potential of the Hellhounds. Later, as Ple-Sen noted, Cleave will be very useful. For attacking from the first turn, Tactics II will be useful for the dogs.
Manics are best suited for Opening Gates. In addition, they can hit quite well and have slightly better defense than the dogs, as it seems to me. Succubi are excellent ranged attackers; they can disable an enemy unit for two turns. Although they are also very fragile.
Now about the Sadists. Not a bad melee unit, yes. But their strength lies in the spikes with which they can attack in all directions. Therefore, the upgrade seems questionable, because the unupgraded ones have spike damage equal to normal damage, and the upgraded ones have a quarter of the damage! So, I don't see the point in such an upgrade.
The other units have not yet been demonstrated in the campaign. But I'm already starting to like the faction; it's not weak at all. The only thing that still makes me sad is the faction ability. After all, you get a very small reinforcement, and you gain ability points incredibly slowly (only for critical hits!)